(1) Field of the Invention
The present invention relates to a motion compensation image coding device and coding method. More particularly, the present invention is concerned with a coding technology for compressing motion picture data, that is, for compensating or predicting a motion in units of each of blocks into which frame data contained in motion picture data is divided, and coding a residual signal, which represents a difference between produced predictive image data and the frame data of the motion picture data, and motion data (that is, a motion vector). The present invention is concerned especially with a technology for searching a motion vector that is included in the compressive coding technology.
(2) Description of the Related Art
In recent years, motion picture coding devices that adopt interpolative coding, motion compensation prediction, orthogonal transformation, quantization, and variable length coding and that are represented by a device conformable to the MPEG (Moving Picture Experts Group) standards have widely prevailed. In motion compensation prediction, a motion is inferred from a reference frame and a predictive residual signal and a motion vector are coded. According to internationally standardized motion picture coding methods including those stipulated by the ITU-T Recommendation H.263 and the ISO/IEC International Standard 14496-2 (MPEG-4 Visual), data of a frame image is divided into 16×16-pixel blocks that are called as macroblocks, and each of the macroblocks is divided into 8×8-pixel blocks. A motion is compensated or predicted in units of a block.
Moreover, according to the H.264/AVC standard stipulating a coding method characterized by a higher compression rate, a macroblock is divided into a plurality of blocks (sub-blocks) each having 16 pixels lined in rows and 8 pixels lined in columns, having 8 pixels lined in rows and 16 pixels lined in columns, having 8 pixels lined in rows and columns alike, having 4 pixels lined in rows and 8 pixels lined in columns, having 8 pixels lined in rows and 4 pixels lined in columns, or having 4 pixels lined in rows and columns alike. A motion is compensated or predicted in each of the blocks. The technology has been disclosed in “Text of International Standard of Joint Video Specification” recommended by the Joint Video Team (JVT) of ISO/IEC MPEG & ITU-T VCEG and “Advanced Video Coding” by the ITU-T Rec. H.264 & ISO/IEC 14496-10 standards (December 2003). The employment of a plurality of sub-blocks makes it possible to predict a motion using any image data including data of a monotonous and motionless image to data of a finely and complexly motional image.
According to the coding methods stipulated by the MPEG-4 and H.264/AVC standards, a motion is compensated or predicted with ½ or ¼ pixel precision. When it says that a motion is compensated or predicted with ½ or ¼ pixel precision, it signifies a method of interpolating virtual pixels located at half or quarter pixel positions using actual pixels located at integer pixel positions, and thus improving precision in motion prediction.
In conventional motion picture data coding, after all macroblocks and sub-blocks are searched for motion data (a motion vector) detectable with decimal pixel precision, an optimal motion vector is determined. Moreover, a technology for varying pixel precision, with which motion vectors are searched, depending on the shape or size of a block has been proposed in, for example, Japanese Patent Laid-Open No. 2004-48552.